An attempt to put a photon counting type X-ray computed tomography apparatus (to be referred to as a photon counting X-ray CT apparatus hereinafter) into practice has been made in the form of expanding a technique for single photon detection (to be referred to as a single photon detection technique hereinafter) in nuclear medicine diagnostic apparatuses such as a single photon emission computed tomography apparatus (to be referred to as a SPECT apparatus hereinafter) and a positron emission computed tomography apparatus (to be referred to as a PET apparatus hereinafter). Single photon detection techniques are roughly categorized into two types.
The first type of single photon detection technique is as follows. First of all, a crystal (scintillator) or the like converts an X-ray photon transmitted through an object into scintillation light. A photodetector such as a photomultiplier tube (to be referred to as a PMT hereinafter) or silicon photomultiplier (to be referred to as an SiPM hereinafter) detects the scintillation light to extract an X-ray photon as an electrical signal. The above method is called an indirect conversion type method.
The second type of single photon detection technique is a method (also called a direct conversion type) which directly converts an X-ray photon transmitted through an object into an electrical signal by using a semiconductor detector. More specifically, a bias voltage is applied in advance to the two electrodes of the semiconductor detector. When an X-ray photon enters the semiconductor detector, the detector internally produces a pair of an electron and a hole. The generated electron and hole are respectively attracted to different electrodes. The electron which has reached the electrode is extracted as an electrical signal.
In either of the above methods, since the integral value of the intensity of the extracted electrical signal (to be referred to as a detection signal hereinafter) is proportional to the energy of the X-ray photon, detection signals are integrated. Integrating detection signals will calculate the energy of individually detected X-ray photon. The difference between the nuclear medicine diagnostic apparatus and the photon counting X-ray CT apparatus is that the flow rate of photons in the photon counting X-ray CT apparatus is much higher than that in the nuclear medicine diagnostic apparatus. In order to reconstruct a medical image by using the photon counting X-ray CT apparatus, it is necessary to perform, for example, single photon detection with respect to 109 photons per mm2 per sec (to be referred to as a count rate hereinafter).
When, however, executing single photon detection for X-ray photons with respect to the above count rate, the aforementioned two types of single photon detection techniques have the following two problems associated with count losses which respectively correspond to them. The problem in the first single photon detection technique is the problem of a count loss due to pileup. Pileup occurs within a typical attenuation time (several nsec) of scintillation when a plurality of X-ray photons enters the scintillator. Pileup is a phenomenon in which a plurality of detection signals respectively corresponding to a plurality of X-ray photons overlap. When pileup occurs, a plurality of X-ray photons are counted as one X-ray photon, resulting in the occurrence of a count loss.
The problem in the second single photon detection technique is the problem of a count loss due to the entrance of X-ray photons to the semiconductor detector during the dead time of the semiconductor detector. The dead time is the time interval from the instant a detection signal is extracted from the semiconductor detector to the instant the semiconductor detector becomes ready for pair production again. When an X-ray photon enters the semiconductor detector in the dead time, no pair production occurs, and hence no X-ray photon is counted. Currently, an attempt has been made to decrease the number of X-ray photons entering the same semiconductor detector in a unit time by lessening the size (pixel size) of the semiconductor detector. In this attempt, however, the maximum count rate has stayed around 106 photons per mm2 per sec.
The problem associated with the above count losses occurs because detection signals with long attenuation time constants are integrated to calculate the energy of an X-ray photon entering the X-ray detector.